1. Field of the Invention
The present invention relates to an exposure apparatus, an exposure method, and a method producing devices. In particular, the present invention relates to an exposure apparatus and an exposure method for transferring a pattern formed on a mask onto a substrate by the aid of a projection optical system. The present invention also relates to a method for producing microdevices based on the use of the exposure apparatus and the exposure method.
2. Description of the Related Art
Various exposure apparatuses have been hitherto used, for example, when semiconductor elements or liquid crystal display elements are produced by means of the photolithography step. At present, a projection exposure apparatus is generally used, in which a pattern formed on a photomask or reticle (hereinafter generally referred to as “reticle”) is transferred via a projection optical system onto a substrate (hereinafter referred to as “photosensitive substrate”, if necessary) such as a wafer or a glass plate applied with a photosensitive material such as photoresist on its surface. In recent years, a reduction projection exposure apparatus (so-called stepper) based on the so-called step-and-repeat system is predominantly used as the projection exposure apparatus, in which a photosensitive substrate is placed on a substrate stage which is movable two-dimensionally, and the photosensitive substrate is moved in a stepwise manner (subjected to stepping) by using the substrate stage to repeat the exposure operation for successively transferring a pattern formed on a reticle onto respective shot areas on the photosensitive substrate.
Recently, a projection exposure apparatus based on the step-and-scan system (scanning type exposure apparatus as described, for example, in Japanese Patent Application Laid-Open No. 7-176468 and U.S. Pat. No. 5,646,413 corresponding thereto), which is obtained by applying modification to the stationary type exposure apparatus such as the stepper, is also used relatively frequently. The projection exposure apparatus based on the step-and-scan system has, for example, the following merits. That is, (1) the projection optical system is easily produced because a large field can be exposed by using a smaller optical system as compared with the stepper, and a high throughput can be expected owing to the decrease in number of shots because a large field is exposed. Further, (2) an averaging effect is obtained owing to relative scanning for the reticle and the wafer with respect to the projection optical system, and it is possible to expect improvement in distortion and depth of focus. Moreover, it is considered that the scanning type projection exposure apparatus will be predominantly used in place of the stepper, because a large field will become essential in accordance with the increase in the degree of integration of the semiconductor element, which is 16 M (mega) bits at present and will become 64 M bits for DRAM, 256 M, and 1 G (giga) bits in future as the progress proceeds along with times.
Such a projection exposure apparatus is principally used as a mass-production machine for semiconductor elements or the like. Therefore, the projection exposure apparatus is necessarily required to have a processing ability such that how many sheets of wafers can be subjected to the exposure process for a certain period of time. That is, the projection exposure apparatus is necessarily required to improve the throughput.
In this context, in the case of the projection exposure apparatus based on the step-and-scan system, when large filed is exposed, the improvement in throughput is expected because the number of shots to be exposed on the wafer is decreased as described above. However, since the exposure is performed during the movement at a constant velocity in accordance with synchronized scanning for the reticle and the wafer, it is necessary to provide acceleration and deceleration areas before and after the constant velocity movement area. If a shot having a size equivalent to a shot size of the stepper is exposed, there is a possibility that the throughput is rather decreased as compared with the stepper.
The outline of the flow of the process in such a projection exposure apparatus is as follows.    (1) At first, a wafer load step is performed, in which a wafer is loaded on a wafer table by using a wafer loader.    (2) Next, a search alignment step is performed, in which the position of the wafer is roughly detected by using a search alignment mechanism. Specifically, the search alignment step is performed, for example, on the basis of the contour of the wafer, or by detecting a search alignment mark on the wafer.    (3) Next, a fine alignment step is performed, in which the position of each of the shot areas on the wafer is accurately determined. In general, the EGA (enhanced global alignment) system is used for the fine alignment step. In this system, a plurality of sample shots included in the wafer are selected beforehand, and positions of alignment marks (wafer marks) affixed to the sample shots are successively measured. Statistical calculation based on, for example, the so-called least square method is performed on the basis of results of the measurement and designed values of the shot array to determine all shot array data on the wafer (see, for example, Japanese Patent Application Laid-Open No. 61-44429 and U.S. Pat. No. 4,780,617 corresponding thereto). In this system, it is possible to relatively accurately determine the coordinate positions-of the respective shot areas at a high throughput.    (4) Next, an exposure step is performed, in which the pattern on the reticle is transferred onto the wafer via the projection optical system while successively positioning the respective shot areas on the wafer to be located at the exposure position on the basis of the coordinate positions of the respective shot areas having been determined in accordance with the EGA system or the like described above and the previously measured baseline amount.    (5) Next, a wafer unload step is performed, in which the wafer on the wafer table having been subjected to the exposure process is unloaded by using a wafer unloader. The wafer unload step is performed simultaneously with the wafer load step (1) described above. That is, a wafer exchange step is constructed by the steps (1) and (5).
As described above, in the conventional projection exposure apparatus, the roughly classified four operations are repeatedly performed by using one wafer stage, i.e., wafer exchange→search alignment→fine alignment→exposure→wafer exchange.
The throughput THOR [sheets/hour] of such a projection exposure apparatus can be represented by the following expression (1) assuming that the wafer exchange time is T1 [sec], the search alignment time is T2 [sec], the fine alignment time is T3 [sec], and the exposure time is T4 [sec].THOR=3600/(T1+T2+T3+T4)  (1)
The operations of T1 to T4 are executed repeatedly and successively (sequentially) as in T1→T2→T3→T4→T1 . . . . Accordingly, if the individual elements ranging from T1 to T4 involve high speeds, then the denominator is decreased, and the throughput THOR can be improved. However, as for T1 (wafer exchange time) and T2 (search alignment time) described above, the effect of improvement is relatively small, because only one operation is performed for one sheet of wafer respectively. As for T3 (fine alignment time), the throughput can be improved if the sampling number of shots is decreased in the case of the use of the EGA system, or if the measurement time for a single shot is shortened. However, on the contrary, the alignment accuracy is deteriorated. Therefore, it is impossible to easily shorten T3.
On the other hand, T4 (exposure time) includes the wafer exposure time and the stepping time for movement between the shots. For example, in the case of the scanning type projection exposure apparatus based on, for example, the step-and-scan system, it is necessary to increase the relative scanning velocity between the reticle and the wafer in an amount corresponding to the reduction of the wafer exposure time. However, it is impossible to easily increase the scanning velocity because the synchronization accuracy is deteriorated.
Important conditions for such a projection exposure apparatus other than those concerning the throughput described above include (1) the resolution, (2) the depth of focus (DOF: Depth of Focus), and (3) the line width control accuracy. Assuming that the exposure wavelength is λ, and the numerical aperture of the projection lens is N.A. (Numerical Aperture), the resolution R is proportional to λ/N.A., and the depth of focus (DOF) is proportional to λ/(N.A.)2.
Therefore, in order to improve the resolution R (in order to decrease the value of R), it is necessary to decrease the exposure wavelength λ, or it is necessary to increase the numerical aperture N.A. Especially, in recent years, semiconductor elements or the like have developed to have high densities, and the device rule is not more than 0.2 μm L/S (line and space). For this reason, a KrF excimer laser is used as an illumination light source in order to perform exposure for the pattern. However, as described above, the degree of integration of the semiconductor element will be necessarily increased in future. Accordingly, it is demanded to develop an apparatus provided with a light source having a wavelength shorter than that of the KrF excimer laser. Representative candidates for the next generation apparatus provided with the light source having the shorter wavelength as described above include, for example, an apparatus having a light source of an ArF excimer laser, and an electron beam exposure apparatus. However, in the case of the electron beam exposure apparatus, an inconvenience arises such that the throughput is extremely lowered as compared with the light beam exposure apparatus.
It is conceived to increase the numerical aperture N.A., as another method to increase the resolution R. However, if N.A. is increased, there is a demerit that DOF of the projection optical system is decreased. DOF can be roughly classified into UDOF (Usable Depth of Focus: a part to be used by user: for example, difference in level of pattern and resist thickness) and the overall focus difference of the apparatus itself. Up to now, UDOF has contributed to DOF in a greater degree. Therefore, the development of the exposure apparatus has been mainly directed to the policy to design those having a large DOF. Those practically used as the technique for increasing DOF include, for example, modified illumination.
By the way, in order to produce a device, it is necessary to form, on a wafer, a pattern obtained by combining, for example, L/S (line and space), isolated L (line), isolated S (space), and CH (contact hole). However, the exposure parameters for performing optimum exposure differ for every shape of the pattern such as L/S and isolated line described above. For this reason, a technique called ED-TREE (except for CH concerning a different reticle) has been hitherto used to determine, as a specification of the exposure apparatus, common exposure parameters (for example, coherence factor σ, N.A., exposure control accuracy, and reticle drawing accuracy) so that the resolution line width is within a predetermined allowable error with respect to a target value, and a predetermined DOF is obtained. However, it is considered that the following technical trend will appear in future.    (1) In accordance with the improvement in process technology (improvement in flatness on the wafer), the difference in pattern level will be progressively lowered, and the resist thickness will be progressively decreased. There will be a possibility that the UDOF may change from an order of 1 μm→not more than 0.4 μm.    (2) The exposure wavelength changes to be short, i.e., g-ray (436 nm)→i-ray (365 nm)→KrF excimer laser (248 nm). However, investigation will be made for only a light source based on the ArF excimer laser (193 nm) and the F2 laser (157 nm) in future. Further technical hurdle is high. Thereafter, the progress will proceed to EB exposure.    (3) It is expected that the scanning exposure such as those based on the step-and-scan system will be predominantly used for the projection exposure apparatus, in place of the stationary exposure such as those based on the step-and-repeat system. The step-and-scan system makes it possible to perform exposure for a large field by using a projection optical system having a small diameter (especially in the scanning direction), in which it is easy to realize high N.A. corresponding thereto.
In the background of the technical trend as described above, the double exposure method is reevaluated as a method for improving the limiting resolution. Trial and investigation are made such that the double exposure method will be used for an ArF exposure apparatus to perform exposure up to those having 0.1 μm L/S. In general, the double exposure method is roughly classified into the following three methods.    (1) L/S's and isolated lines having different exposure parameters are formed on different reticles, and the exposure is doubly performed for each of them on an identical wafer under an optimum exposure condition.    (2) When the phase shift method or the like is introduced, L/S has a higher limiting resolution at an identical DOF as compared with the isolated line. By utilizing this fact, all patterns are formed with L/S's by using the first reticle, and L/S's are curtailed for the second reticle to form the isolated lines.    (3) In general, when the isolated line is used, a high resolution can be obtained with a small N.A. as compared with L/S (however, DOF is decreased). Accordingly, all patterns are formed with isolated lines, and the isolated lines, which are formed by using the first and second reticles respectively, are combined to form L/S's.
The double exposure method described above has two effects of the improvement in resolution and the improvement in DOF.
However, in the double exposure method, the exposure process must be performed several times by using a plurality of reticles. Therefore, inconveniences arise in that the exposure time (T4) is not less than two-fold as compared with the conventional apparatus, and the throughput is greatly deteriorated. For this reason, actually, the double exposure method has not been investigated so earnestly. The improvement in resolution and depth of focus (DOF) has been hitherto made by means of, for example, the use of a short exposure wavelength, modified illumination, and phase shift reticle.
However, it is doubtless that the realization of exposure up to 0.1 μm L/S based on the use of the aforementioned double exposure method for the ArF exposure apparatus makes a prevailing choice for the development of the next generation machine aimed at mass production of 256 M bits or 1 G bits DRAM. It has been demanded to develop a new technique for improving the throughput in order to solve the problem of the double exposure method as the bottleneck for the purpose as described above.
In view of the above, it is considered that if two or more of the four operations described above, i.e., the wafer exchange, the search alignment, the fine alignment, and the exposure operation can be simultaneously dealt with in a concurrent manner even partially, the throughput can be improved as compared with the case in which the four operations are performed in the sequential manner. For this purpose, it is premised that a plurality of substrate stages are provided. Theoretically, this arrangement is considered to be easy. However, actually, there are an extremely large number of problems to be solved in order to provide a plurality of substrate stages and exhibit a sufficient effect thereby. For example, a case may be assumed, in which the scanning exposure is performed for a substrate on a first substrate stage, during which the alignment is performed for a substrate on a second substrate stage. The reaction force, which is caused by the acceleration or deceleration of the first substrate stage and a first reticle stage, acts as disturbance on the second stage, and it causes a factor of the alignment error. In order to realize the highly accurate overlay, it is necessary that the alignment is executed for the substrate on the identical substrate stage, and then the exposure is performed by executing the position adjustment for the pattern on the mask and the photosensitive substrate by using the result of the alignment. Therefore, no practical solution can be obtained by a simple countermeasure, i.e., for example, if one of the two substrate stages is exclusively used for the exposure, and the other is exclusively used for the alignment.
The present invention has been made taking the circumstances as described above into consideration, a first object of which is to provide an exposure apparatus and an exposure method which make it possible to improve the throughput.
A second object of the present invention is to provide an exposure apparatus and an exposure method which make it possible to improve the throughput and realize the highly accurate exposure for a fine pattern.
A third object of the present invention is to provide a method for producing devices, which makes it possible to produce a microdevice at low cost.